I was told by a flyer at our field that as you move the CG back towards the tail, the stall speed decreases. Is this right? This is just the opposite of what I thought.

In a small way he's right. Moving the CG back moves part of the lifting load to the stabilizer from the wing. Or in the case of extremely forward CG's it reduces the download at the stabilizer and lessens the work needed from the wing. BUt for normal CG locations the effect is mnimal and you'd be hard pressed to notice the difference.

Or he may be reffering to the easier job of keeping the nose up and "hanging on the prop" which makes it appear to be delaying the stall. BUt that's not really delaying the stall since you're then relying on the engine thrust line being pointed up to provide some relief from the weight the wing is carrying. And that's CHEATING! ! ! ! !

If its far enough back that you're getting significant lift from the horizontal stab/elevators, then the loading on the wing goes down correspondingly. Of course that may be a virtually uncontrollable unstable configuration for a "normal" airplane ;->

Many gliders and FF power rigs put the CG near the TE or even behind and use a heavily lifting horizontal stab to compensate.

And a wing will always stall at an angle to the relative airflow, so cg, weight and speed don't actually affect that angle, merely how easy it is to reach that angle.
Evan, W.B #12.

The lifting airfoil on the tail doesn't compensate for the CG being that far back. It's the area of the stabilizer combined with a long tail moment that allows the CG to be at these extreme locations. Using a lifting section on the stabilizer just allows the tail to provide the lift it would do so anyway in a more efficient manner. The CG isn't that far aft because of the airfoil but the airfoil is shaped that way because the CG is that far back.

Vic,

What I think your friend was trying to say is that a properly balanced plane can fly slower than one that is nose heavy.
If the plane is nose heavy, the tail needs to pull downward to keep the nose up, thusly increasing the "work" that the mainwing needs to perform, either by increasing the angle of attack, or airspeed to increase lift.
Since any given wing will stall at the the same angle of attack no matter where the CG is, it becomes a matter of having to move the air over the wing faster to counteract the downward force needed by the tail pulling down trying to keep the nose up.

Bob

I've seen and flown an RC F-15 Jet that was so perfectly balanced on the wing that it would literally teeter totter at some air speeds across the CG.

I definatly would not recommend a aft CG. Not GOOOD!!!

I've just done some pencil/paper work to get a better understanding of what I can do with a Sukhoi31 of mine. It's flight has been tuned with CG moves for 5-6 months now. It started out with the CG around 3.5" back and has it back about 4.3" now.

It very definitely shows the ability to more quickly do snappy maneuvers, where the wing is stalled to execute the trick. But be darened if I've noticed the slower stall speeds. Why?

Because for almost every one of us, we really can't tell the difference between 15mph and 18mph. Yeah, moving the CG changes those critical speeds but darned if any of us can usually see the difference. If we could, there wouldn't be a single case of landing gear being driven up into the fuselage on those "drop from 10feet" landings that we see almost every weekend at the flying field.

The CG on the Sukhoi can be moved over an inch and the sucker still seems to land about the same speed. Why? Because the wind speeds it sees on any given landing vary more than how much the Vmin changes from CG changes. And most of us simply can't make accurate use of a 3mph difference in Vmin. Yeah, it's there, but who can see it?

It's one of those things that's kewl to discuss, but not much good for more than that.

Da Rock,

If you really want to see what your plane is stalling at, you can get http://www.eagletreesystems.com/Plane/plane.html
I have an early version, and have used it in a"troubled" plane. I helped me in tuning my Ultra Stick. It is was both stable when you were gentle with it, and snappy when you wanted it to be when I was done with it. (I used past tense because I had dumb-thumbs and augered it in one fine morning)
I do not have the live feed, so I have to take a few runs, hook it up to my laptop, and analyze the data.
With the Eagle Tree Systems Flight data recorder, you don't have to guess what the plane is doing... It's right there. Graphs of flight speed, stick inputs, artificial horizon, altitude, and rate of climb, etc.

Bob

I really don't care what exact speed any of my models stall for a very good reason. I'm no better at seeing exact speed in the air than any human. And since the winds encountered are a complete unknown, and almost always present, it's pretty much a waste of time to know the exact speeds.

Changes in stall speeds aren't going to be great. Matter of fact, the changes in speed possible aren't going to be great enough to actually be significant enough to be useful. But it's really fairly simple to tune elevator throws to fly the model significantly safer on low rates and energetic enough on high rates.

I really don't care if a change will theoretically increase the stall speed from say 16mph to 18mph. Almost nobody can tell the difference in those two speeds in real life. And while attempting that fine a judgement, figure out what the wind out in the approach area is and what it's direction is and what that combination means to the 2mph improvement. We can't see speed, or wind, or the difference in AOA that spells stall well enough for any of the numbers to matter. So I basically don't use them beyond finding the safe range for a CG for the maiden. Or to check CG changes later when I'm sorting the airplane.

But for people who like to know all the numbers, it's really kewl that we do have apps and hardware that give 'em to us. Thanks for the link.

I've found that what daRock is saying is truly the case. Wind speeds render stuff like this that relates to small changes useless. Yeah, it's there but to use an old radio phrase the signal is lost in the noise from all the other larger effects that are changing to a far greater degree and constantly.

I've found that I pick up on my model's near stall condition by the response to control inputs that it has and to some extent how it's flying. If it's slow to respond and mushy then it's near stall. If it's moderately responsive and flying in the groove then it's safely into the cruise range. If it's overly sensitive to the controls then it's flying high in its speed range. If it starts fluttering or parts begin to shed then it's time to power back...

Hell yeah! 3mph, you can hardly notice 3mph is a real aircraft (i am a flight instructor) so you sure as hell cant tell in an RC aircraft.

Moving the CG towards the tail will have an insignificant effect on stall speed; but it will have a significant effect on the stability of the airplane.
There will be a change in stall speed with weight change regardless of the CG location.

feihu

There is no such thing as stall speed, only stall angle. If you move the cg back you can reach that stall angle at a higher airspeed, so in my opinion your friend is wrong. Remember, nose heavy aeroplanes don't fly very well. Tail heavy aeroplane don't fly very long.
John

Aft CG REDUCES stall speed, not increases stall speed as you indicated. Stall angle is reached at different speeds due to, but not limited to, wing loading and CG.

Please see http://www.copanational.org/non-memb...etyPPOct99.htm
Here is an excerpt from this page -

AND

Bob

I'll just throw this into the soup:

Many years ago I built a Goldberg Cub, after a while it met its expiration date.

A year or two later, I built another one. I used the same engine, same battery location, etc. and I didn't bother to check the CG. (Yea, I was Young and stupid and I figured since everything was the same the CG would be close enough)

Well I could never land that plane properly. No matter what I did, it always came in like a dart and wouldn't slow down.

Then, one day I did a spin with it, and it wouldn't spin - it just sort of spiraled down. Now I KNEW this was a sign of being nose-heavy, so I checked the CG and sure enough, it was nose heavy.

So I moved the CG back to the proper range and from that day on, it floated in gently on landings.

Since that time I have witnessed this on several of other people's planes. So now, when someone tells me their plane lands hot, the first thing I say is, "Check your CG"

Bob, please read what I said. A rearward cg means that the elevator has more power. This in turn means that you can reach the stalling angle more easily. This is the sole reason why we often advise modellers to make their first flight with the CG a little forward, as its safer.
I also said there is no "stalling speed". To clarify, the speed an aerofoil stalls is not constant (I think we are agreeing here). The stalling angle of an aerofoil is a constant.
Regards,

john

Hi John,

Yes, I agree with your last post. Sorry for being nit-picky.
I just don't want people to misunderstand what is being offered as correct information.
Sometimes I do it myself, offer incorrect information, not intentionall, but by miscommunication.
I am not doubting your knowledge in aeronautics, but statements like "If you move the cg back you can reach that stall angle at a higher airspeed" are mis-leading.
It seems as though it was stated that the aft CG increases stall speed.
A stall angle can be reached at any speed above the velocity where the wing ceases to produce lift, (stall), if you have enough elevator authority. It is called an accellerated maneuver stall.

Oh, one more thing to the mix ----
If you have enough engine power, you can support more, even all of the weight of the aircraft with propthrust, reducing the amount of weight the wings have to support, (lowering wingloading). This lowers the speed required to reach the stall angle in normal level flight.

Thanks John.


Bob

I had played with CG on my 4*40. Flew it for a couple years with the cg at the middle of the recommended range. Easy to fly and land.

I started to move the cg back a bit at a time. It became easier to land, landed slower, flew inverted with less up elevator.

New discovery, cool! If a little is good more is better right! I knew I had gone too far now. The plane got slower during the flare on landing. But now it would stall and snap and bang to the runway. If I was able to time the stall at the same time the main gear touched down, no problem. In summary, it would fly slower but the stall was harder/faster when it stalled.

With the CG more in the normal range, the stall was softer and it doesn't bang to the runway. I moved the cg forward to eliminate the hard stall and called it good.

That is a good example of what i was saying.

Correct information is subjective, depending on the use to which it will be put. Technically it is correct to say that you can fly a plane slower with the cg at its rearward limit (which is in itself subjective), but how useful is it to know this in isolation? Indeed, in isolation this fact may lead a pilot to believe that a rearward cg is safer.
Take the following scenario: You are flying you Cessna xyz straight and level in perfect conditions at say 5000 feet. You have a very large passenger in the front seat with you. This means the aircraft is flying at its forward most CG limit. Also the aircraft has a very accurate and well calibrated ASI, in good working order. You set power to idle and ease back on the stick until the aircraft stalls at 75kts IAS.
You know push your passenger out The aircraft is now more tail heavy and at its rearmost cg limit. You try the same experiment and the aircraft now stalls straight and true at 70 kts IAS, 5 Kts slower than before. Great! It is technically interesting and proves the theory, but in which way is this knowledge on its own of practical benefit to the model pilot?
I my experience, if I am flying a model that "feels" tail heavy, I trim it down to make it fly faster since I have NO IDEA what its airspeed is. Moreover, any departure from the specific set of condition will change the speed that the aircraft will stall, and even the mode in which it will stall. Change the wing loading, angle of bank, flap or slat position etc, aileron or rudder input, all will change the speed of the stall.
Since the model pilot cannot be accurately aware of many of these conditions, he must fly the model by visual cues combined with being aware of his own stick inputs to judge when then model will stall, and by so doing avoid it. This is the model pilots version of the truth!
WMB has provided us a good example of what can happen if you move the CG too far back, and this is, I suspect with a lightly loaded model.
I reiterate that nose heavy aeroplanes do not fly very well, but tail heavy aeroplanes do not fly very long.
John

You got excellent points there. And in the modeling world, too many have come to believe that a nose heavy model is safer. It is not.

Add this to those good points.

There probably aren't 20 model flyers in the world who can make use of the difference in speed IF it happens. First off, nobody can tell the difference between the model flying at 18mph and it flying at 20mph. And that's about how much the change in speed is going to be. Secondly, there aren't too many modelers who can accurately take their model airplane right up to the max CL and hold it there, just under the AOA of the stall. And thirdly, no modeler alive can see what the windspeed is around the models, so flying at the last few degrees of AOA is just not going to happen safely, since gusting is one of the major problems to safe landings.

Wringing the last bit of performance out of any model is fun, but this one is simply out of the model flyers' capabilities. We can gain a few mph for sure, but using it accurately just isn't going to happen. Using it at all really isn't either.

In practice, the best way to get a model to land more slowly with safety is to tune the engine to idle more slowly with dependability and to put a flatter pitch prop on the sucker. And even that is degraded when the CG is farther forward than optimum. Get the CG adjusted to the optimum for control and then work on the engine.

Well put, Da Rock.

Engine tuning and prop pitch are paramount in slow flying model planes.
The origin of this post was if the CG was towards the rear, the plane would fly slower. This statement is true to a point.
Move the CG too far aft, and the plane flies once.
Move the CG too far forwards, and the plane won't get off the ground.
Set the CG at the manufactures specs, play with it within the guidelines of the plans, (more if your wallet is fat - $$$), and see what happens.
Experience is more than anything you can read in this forum.

But PLEASE - These guys ARE experienced. Take their advise wisely.

Rearward CG will make the plane fly slower, if it is within the model specs. Anything farther can result in a white knuckle ride, or disaster... [:@]

Bob